Tsetse flies are important vectors of disease for both humans and livestock, transmitting the protozoan parasites African trypanosomes. In addition, they have been shown to carry a rich assemblage of bacterial symbionts; three different intracellular organisms have been described to date. The first nutritionally obligatory symbiont is harbored in specialized epithelial cells (bacteriocytes) in foregut. The second facultative organism lives in midgut and has been suggested to affect the vector competence of tsetse while the third symbiont is in reproductive tissues and has been shown to induce various reproductive anomalies in other insects. As such, these intracellular symbionts have the capability of exerting significant physiological control over their insect host. This proposal will investigate the potential role midgut symbionts play in tsetse vector competence. Susceptible flies will be cured specifically of their midgut infections using a symbiont specific antibody feeding approach which does not affect the viability of the obligatory bacteriocyte-associated intracellular organisms. In addition, symbionts, from susceptible fly species will be introduced into refractory lines and the susceptibility of the generated lines will be evaluated. In field based experiments, the gut symbionts of natural populations will be identified. The midgut symbionts have been cultured, transformed and reintroduced into flies. This symbiont transformation system will be utilized to express and evaluate activities in the gut that may affect transmission of trypanosomes. Specifically, studies will concentrate on the role of the chitinase enzyme for parasite transmission. Chitinase will be overexpressed in the gut and its effect in susceptibility will be evaluated by transmission experiments. These experiments will also address the role of the peritrophic matrix in trypanosome transmission. The basic knowledge obtained will strengthen our understanding of tsetse biology and will also provide information relevant to other important disease vectors which harbor similar symbionts (e.g., ticks, fleas, reduviid bugs etc.).